Stabilizing chlorinated hydrocarbons



Patented Feb. 15, 1938 UNITED STATES STABILIZING CHLggggATED HYDROCAR- Whitfield Price, Charleston, W. Va, assignor, by

mesne assignments,

to Westvaco Chlorine Products Corporation, New York, N. Y., a corporation of Delaware No Drawing. Application July 20, 1936, Serial No. 91,620

14 Claims.

This invention relates to stabilizing chlorinated hydrocarbons; and it comprises a method of stabilizing trichlorethylene against deteriorative internal changes by dissolving a modicum of an alkyl thiocyanate, most advantageously methyl thiocyanate, therein; a similar method being applicable to other chlorinated. hydrocarbons; and it also comprises a chlorinated hydrocarbon showing on test the presence of a volatile thiocyanate ester; all as more fully hereinafter set forth and as claimed.

Chlorinated hydrocarbons'are in large commercial use because of their uninflammability, convenient boiling points and good properties as solvents for grease, resins, rubber, etc. Trichlorethylene in particular finds much favor. It. is a substance of convenient boiling point, around 87 0. Most of these chlorinated hydrocarbons are bodies of good stability, insofar as attack by reagents is concerned. They are quite inert chemically. All, however, have some tendency to undergo internal changes in time, this being particularly marked in the case of trichlorethylene. The nature of these changes, that is, the chemical mechanism involved, is not quite definitely known, but in all cases there is a development of acidity. Exposure to both light and air favor these changes; neither singly having as much efiect as the two together. The particular susceptibility of trichlorethylene may be due .to its unsaturated character.

Various additions to these chlorinated hydrocarbons have been proposed in enhancing their stability and inhibiting internal changes. To some extent, these are eflective, but the difficulty arises that any inhibitor in the chlorinated hydrocarbons must itself withstand trying conditions; it must itself be stable. In many of the commercial uses of these chlorinated hydrocarbons, the same portion of solvent may be kept at a boiling temperature indefinitely in the presence of various, impurities. For example, in a current method of degreasing metal and other articles, the article is passed through a body of vapor of trichlorethylene overlying a body of boiling liquid. Condensed solvent trickles off the article, carrying grease with it, and the solvent is revaporized to serve anew. In this operation, a limited amount of trichlorethylene at its boiling temperature is continually passing back and forth between the liquid and the vapor states and the stabilizer must persist over an indefinitely long time.

I have found that various thiocyanate esters, or sulfocyanides, containing the radical .SCN,

are particularly effective stabilizers for chlorinated hydrocarbons. The reason for this is not obvious, but the fact is as stated. For practical reasons, alkyl thiocyanates are employed, these all being volatile materials. The isothiocyanates are serviceable, albeit not better than the thiccyanates. In these compounds, there is a characteristic nucleus. They contain a single hydrocarbon group connected by a single bond to the group SCN and the latter group in turn has the carbon atom thereof attached to a single nitrogen atom by at least two bonds; in the isothiocyanates the carbon and nitrogen are linked by a double bond and in the thiocyanates by a triple bond. The best of these alkyl thiocyanates is the methyl ester, CH3. SCN. Methyl thiocyanate has the lowest boiling point of these alkyl estershaving a boiling point between and 131 C. I have found it most advantageous for my purposes, partly because of this fact and partly because it seems to have a characteristic stabilizing effect greater than that of the other alkyl thiocyanates. which I have used and found desirable are ethyl thiocyanate, isopropyl thiocyanate, butyl thiocyanate, methyl isothi cyanate, ethyl isothiocy anate, and allyl isothidgyanate.

In degreasing with trichlorethylene, all these esters stabilize the boiling liquid and stabilization is not impaired by accumulating impurities. When the trichlorethylene is recovered by distillation the thiocyanate can also be distilled over.

While the stabilizing efiect of these thiocyanates is most marked with trichlorethylene, they are also advantageous with carbon tetrachloride, chloroform, ethylene dichloride, the tetrachlorethanes and even with chlorinated aromatics.

Ordinary unstabilized trichlorethylene. in a loosely stoppered bottle exposed to diffused daylight in a few hours becomes heavily acid on test; an aqueous extract made with neutral water exhibits enough acidity to titrate. Phenolphthalein is a suitable indicator for this purpose. With a loose stopper, a little access of air occurs, but there is not much-evaporation. On the other hand, a similar test'made with the same tri-. chlorethylene containing a trace of methyl thiocyanate does not develop appreciable acidity after long standing.

In accelerated tests of similar character but under more drastic conditions as regards illumination and access of air, trlchlorethylene containing a little methyl thiocyanate does not develop substantial acidity over long periods of .time.

Some of these tests simulate in a few days what Among these other thiocyanates would happen in years of normal storage at the ordinary temperature.

In one test a quantity of the liquid is placed in the flask of a Soxhlet apparatus. The compartment of the Soxhlet which acts as a receiver for condensate, and which is equipped to siphon back into the flask at intervals, contains a piece of copper. A second piece of copper is placed in the flask itself and is of such dimensions that part of its surface is below the liquid and part in the vapor space above. Corrosion of the copper is evidence of decomposition of the solvent. In this particular accelerated test conditions are like those occurring in a degreaser system.

Heat is applied to the flask at such a rate that mild refluxing occurs, so that over a period of time, such as an hour or two, the siphon compartment fills up and discharges back into the flask, Precautions are taken that the equipment stands in diflused light rather than direct sunlight. Acidity determinations are made at intervals on the liquid at a time when the siphon compartment is empty. Oneof the features of this test is the fact that liquid is distilled out of the flask and condensed, and is exposed to the action of light and oxygen away from the main body. It is assumed that certain stabilizers will follow the vapor and condense with it so as to stabilize the liquid in the siphon chamber, while others of lower vapor pressure will remain in the flask while unstabilized vapor passes into the siphon compartment.

Using this accelerated test, a sample of trichlorethylene containing 0.004 per cent methyl thiocyanate developed no substantial acidity in a period of 72 days, whereas unstabilized trichlorethylene in the same test will show high acidity in a very short time. In another accelerated test, a sample of trichlorethylene without stabilizer in 2 days developed an acidity requiring 550 drops of hundredth normal NaOH for neutralization, per 10 cc. of the sample. The sample was extracted with neutral water and the water titrated. On the other hand, the same trichlorethylene with an addition of 0.01 per cent methyl thiocyanate, after 4 days exposure, had an acidity requiring only 7 drops per 10 cc., while trichlorethylene containing 0.05 per cent isopropyl thiocyanate, after 3 days exposure developed only an acidity of the same order of magnitude, requiring 8 drops per 10 cc. for neutralization in lieu of '7 drops.

Similar results are given by other chlorinated hydrocarbons, such as, perchlorethylene (CClzICClz). Ethylene dichloride is a solvent much in use and it shows some tendency to decomposition under drastic industrial conditions. A sample of a particular ethylene dichloride of high commercial grade subjected to an accelerated test for 10 days developed in 10 cc. an acidity requiring 260 drops of the hundredth normal caustic soda, for neutralization while the same ethylene dichloride with an addition of 0.02 per cent methyl thiocyanate, after 10 days exposure. developed an acidity requiring only 10 drops per 10 cc. for neutralization.

'As stated, the chlorinated hydrocarbons are bodies of tolerably inert character. In a pure neutral anhydrous state, they do not affect, nor are they affected by, the common metals. But with any .development of acidity, the situation alters. It is, 01' course, of enormous practical importance to maintain the inert character of these solvents, this being especially true with the solvents used in degreasing metals, where they are in constant contact with the metals. In the case of trichlorethylene, experience has shown that with a little dissolved methyl thiocyanate, the trichlorethylene becomes stable indefinitely long in the degreaser, whatever the character of the metal used in making the apparatus or treated for degreasing.

While I have spoken of the stabilizing effect of methyl thiocyanate and the other thiocyanates used with pure liquid: chlorinated hydrocarbons, the same stabilizing eflect is secured in admixtures; either of chlorinated hydrocarbons with each other or with other compounding materials.

In chlorinated materials containing the thiocyanates of the present invention its presence can be recognized by eflecting saponification of a sample of the chlorinated hydrocarbon with caustic soda, thereby making a thiocyanate. This can be identified by the usual ferric iron test.

What I claim is:-

1. 'Irichlorethylene stabilized by the presence of a small amount of an alkyl thiocyanate.

2. The composition of claim 1 in which the alkyl thiocyanate is methyl thiocyanate.

3. A chlorinated hydrocarbon stabilized by the presence therein of a fraction of a per cent of an alkyl thiocyanate.

4. The composition of claim 3 wherein the chlorinated hydrocarbon is ethylene dichloride.

5. The composition of claim 3 'wherein the chlorinated hydrocarbon is trichlorethylene.

6. The composition oi. claim 3 wherein the chlorinated hydrocarbon is perchlorethylene.

'7. In the stabilization of chlorinated hydrocarbons against deteriorative internal changes, the process which comprises dissolving therein an alkyl thiocyanate in an amount corresponding to a fraction of a per cent by weight.

8. The process of claim 'Iwherein the alkyl thiocyanate is methyl thiocyanate.

9. The process of claim 7 wherein the addition of thiocyanates is in amounts between 0.001 per cent and 1.00 per cent.

10. The process of claim 7 wherein the alkyl thiocyanate is methyl thiocyanate.

11. The composition of claim 1 wherein the alkyl thiocyanate is the normal thiocyanate.

12. The composition of claim 1 wherein the alkyl thiocyanate is an iso-cyanate.

13. As a composition of matter, a substantially neutral, stable liquid comprising a volatile liquid chlorinated hydrocarbon containing dissolved therein between 0.001 to 1.000 per cent by weight of a thiocyanate having the following structure:

- RS-C EN wherein R is an alkyl group, the amount of the said thiocyanate being sufllcient to stabilize the structure:

R-N=C=S wherein R isan alkyl group, the amount of the said thiocyanate being suflicient to stabilize the said chlorinatedhydrocarbon against deteriorative internal changes.

WHITFmD PRICE. 

